Float glass manufacture apparatus

ABSTRACT

In the float process for the manufacture of glass, an alkali metal is removed from the bath of molten metal by contacting the metal with a body of material which dissolves alkali metal oxide and connecting a D.C. source across the body to establish a field therein such that alkali metal ions are drawn into the body.

United States Patent Loukes et al.

FLOAT GLASS MANUFACTURE APPARATUS David Gordon Loukes, Prescot; AlanEdwards, Widnes, both of England Pilkington Brothers Limited, Liverpool,England Filed: Mar. 13, 1969 Appl. No.: 807,058

Inventors:

Assignee:

Foreign Application Priority Data [56] References Cited UNITED STATESPATENTS 3,330,635 7/1967 Loukes et al. ..65/l82 3,330,637 7/1967 Loukeset al. 3,480,420 I 1/l969 Loukes et al. 3,492,l07 l/l970 Simpson etal... 3,467,508 9/1969 Loukes et al. ..65/99 FOREIGN PATENTS ORAPPLICATIONS 183,952 8/1966 U.S.S.R. ..204/68 Primary Examiner-Frank W.Miga Attorney-Morrison, Kennedy and Campbell [57] ABSTRACT In the floatprocess for the manufacture of glass, an alkali metal is removed fromthe bath of molten metal by contacting the metal with a body of materialwhich dissolves alkali metal oxide and connecting a DC. source acrossthe body to establish a field therein such that alkali metal ions aredrawn into the body.

5 Claims, 3 Drawing Figures Mar. 21, 1968 Great Britain ..l3,789/68 US.Cl ..65/l82, 65/65, 65/99, 204/68, 75/85 V Int. Cl. ..C03b 18/00 Fieldof Search ..65/27, 99, I82, I68; 204/68; 75/85 0. C. Sou/C8 PatentedApril 25, 1972 tlorney;

FLOAT GLASS MANUFACTUREAPPARATUS BACKGROUND OF THE INVENTION ticular tothe float process and apparatus for the production of flat glass inwhich glass is supported on a bath of molten metal. A molten metal bathfor this purpose is'preferably abath of molten tin or a molten tin alloyhaving a specific gravity greater than that of the glass and in whichtin predominates.

The bath of molten metal is usually contained in a.tank structure havinga refractory lining comprising, for example, refractory blocks held inan outer metal casing. lt'has been found that in use of such apparatusglassy accretions tend to form on the surfaces of the refractory liningin contact with the molten metal bath. If such accretions becomedetached from the bath lining they rise through themolten metal to thesurface of the bath, where'their presence can be detrimental to thequality of the glass formed on thebath, particularly in the hotterregions of the latter, where the temperature may be SUMMARY According tothe invention, apparatus for use in the production of glass comprises atank structure having a bath of mo]- ten metal upon which glass issupported, and means for removingan alkali metal form the bathcomprising'a body of material which dissolves an alkali metal oxide andwhich is in contact with the molten metal of the bath, and a directcurrent source connected across the body so that an electric field isestablished in the body, said field being directed away from the surfaceof the body which is in contact with the molten metal to cause alkalimetal ions to be drawn into the body away from said surface.

Preferably electrical connections are made from thepositive and negativeterminals of the direct current source to the molten metal bath and tothe upper surface of the body respectively. Inthis case the electricalconnection to the body is preferably effected through a body of moltenmetal provided on the upper surface of'the body.

In a preferred embodiment of the invention the said body comprisesrefractory material. For example the body may comprise part of arefractory lining of the tank structurewhich is immersed in the moltenmetal of thebath.

In one preferred embodiment of the invention the body is maintained incontact with a part of the upper surface of the molten metal bath whichis not covered by the glass. In an alternative preferred embodimentmeans are provided for circulating the molten metal of the bath througha chamber communicating with the tank structure, the said body beingdisposed in said chamber in contact with said molten metal.

The invention also comprehends a method of producing glass in which theglass is supported on the upper surface of a molten metal bath,characterized by maintaining in contact with the molten metal of thebath a body of material which dissolves an alkali metal oxide from thebath, and establishing an electric field being established in the saidbody in adirection away from the surface thereof in contact-with thesaid molten metal so as to cause alkali metal ions to be drawn into=thebody away from said surface, whereby'the alkali metal content of thebath is reduced.

The invention further comprehends glass produced by the methodhereinbefore described.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a partial diagrammatic crosssection of a glass treatment tank incorporating one embodiment of theinvention, and

FIG. 2 is a diagrammatic sectional elevation of part of an apparatusin'accordance with another embodiment of the invention, and

FIG. 3 is a plan view of the apparatus of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a diagrammatictransverse cross section ofa tank structure 1 in which glass in the formof a continuous ribbon 2 is manufactured and/or treated. The tankstructure 1 is elongated in a direction perpendicular to the plans ofFIG. 1 and contains a bath of molten metal 3, usually tin or a tinalloy, upon which the glass ribbon 2 floats. The glass is supplied tothesurface of the bath 3 at one end of the tank in a molten state from aglass melting tank through a regulating tweel (not shown) and the ribbonof glass 2 which forms on the bath 3 is advanced along the tank, thetemperature of the glass being controlled by means of suitable heaters(not shown) in and above the bath 3 so that at the other end of the baththe ribbon 2 has hardened sufficiently to be withdrawn from the surfaceof the bath 3. Typically the temperature of the glass ribbon 2 at theinlet end of the bath 3 is of the order of l,000 C, while at its removalfrom the bath the ribbon temperature may be of the order of 600 C.

The bath 3 is enclosed by a roof structure 4 which defines'a plenumchamber 5 over the glass ribbon 2. An inert gaseous atmosphere, forexample, of nitrogen, is maintained in the plenum chamber 5 to preventoxidation of the exposed surfaces of the molten metal bath 3.

The tank structure 1 has a refractory lining constituted by refractoryblocks 6 defining the walls and base of the tank and enclosed in a metalcasing 7.

In operation of the apparatus, an alkali metal, for example, sodium,supposedly in the form of soda (sodium oxide, Na O), in the glass ribbon2 undergoes a cation exchange reaction with stannous oxide in the bath3, resulting in a proportion of soda entering the bath 3. There is atendency, which is more acute at the hot inlet end of the bath, for thesoda thus present in the molten metal bath 3 to form a glaze on thesurface of the refractory lining blocks 6. Such glazing of therefractory lining has been found to lead to two undesirable effects onthe glass ribbon 2 being formed:

i. The glazing of the refractory lining renders the lining impervious togases, such as hydrogen which come out of solution in the tin of thebath 3. Consequently such gases form bubbles which rise through the bath3 to form openbottomed bubbles on the lower face of the glass ribbon 2.

ii. Glassy accretions forming the glaze on the refractory lining. fromtime to time break away from the lining, whereupon they rise through thebath 3 and adhere to the lower face of the ribbon 2. The adhering glazeresults in a defect resembling a tadpole in appearance, the tail of thetadpole frequently being several feet along.

A glaze also tends to arise on the refractory tank liningwheneffecting-a surface treatment of a glass ribbon on a molten metalbath, particularly by electrolytic treatment, where anelectric currentis passed through the glass and through the supporting molten metalbath. Sodium metal will be present in the bath as a result of theelectrolytic action on the glass, and

this sodium has a deoxidant effect on stannous oxide in the bath, whicheffect may be represented by:

m") Sno z mm The resulting soda (Na O) in the bath produces glazing ofthe refractory lining of the bath as hereinbefore described.

The present invention prevents or inhibits the formation of glaze on therefractory tank lining by removing soda or sodium metal from the moltenmetal bath.

FIG. 1 illustrates one method of carrying out the invention, thecross-section of the tank structure 1 being taken near the inlet end ofthe bath, where the temperature is about 950 C. It will be seen that theglass ribbon 2 does not occupy the entire width of the bath surface. Afloating body of refractory material 8 lies on part of the uncoveredsurface of the bath 3 alongside theribbon 2. The refractory body 8comprises an elongated block extending'parallel to the adjacent edge ofthe ribbon 2 and typically up to 1 inch in thickness. A suitablerefractory material-for the-body 8 is silica (SiO A recess 9 is formedin the upper surface of the body 8 and a pool of molten conductivematerial 10, conveniently tin, is contained in the recess 9. A directcurrent source 11 has a negative terminal connected to the pool 10 and apositive terminal connected to the molten metal bath 3, so that anelectric field E is established in the refractory body 8, directed awayfrom the surfaces thereof contacting the bath. Typically the source 11supplies 20 amps. at 100 volts and the surface area of the body 8contacting the bath 3 is 20 square feet, so that a current density of 1amp per square foot passes through the body 8.

Soda from the tin in the bath 3 dissolves at the surface of therefractory body 8. Under the influence of the applied field E, sodiumions are drawn into the body 8, oxygen being released at the surface ofthe bath 3, where it forms stannous oxide with the tin of the bath. Thesodium ions are neutralized at the pool 10, where sodium metal isreleased.

The effect of the continuous removal of sodium ions through the body 8is to maintain the lower surface of the body 8, in contact with the bath3, free of soda, thereby promoting the continuous withdrawal anddissolution of further soda from the bath 3.

The sodium metal released into the molten tin pool 10 either volatizesdirectly at the temperature of the body 8 (950 C), the boiling point ofsodium being 880 C, or is oxidized to Na O by residual oxygen in theplenum chamber 5; such Na O either vaporizes or dissolves in the uppersurface of the body 8. A vent (not shown) for the removal of sodium orsoda (Na O) vapor is conveniently provided.

The molten tin pool can if desired be changed periodically. It ishowever, preferred to permit vaporization of sodium and soda asdescribed above while allowing the upper surface of the refractory block8 itself to become glazed by dissolving soda. In operation, therefore,glaze is formed in the body 8 and eats its way downwardly through thebody 8 from the upper surface.

The movement of soda molecules through the molten tin bath to thepurification zone occupied by the refractory body 8 is determined by thenatural flow of tin in the bath 3. By using an electro-chemical systemof the kind shown which is separate from the glass ribbon 2 and anyelectrolytic treatment applied thereto, a high current efficiency forthe removal of soda from the bath is obtained. The reason for this isthat the surface of the refractory body 8 in contact with the bath 3rapidly becomes highly polarized with respect to tin ions (Sn so thatthe major contribution to the current flow through the body 8 is due tothe removal of soda from the bath.

In the course of time the refractory body 8 acquires so much soda (Na O)that it softens to the extent that the sodium-rich pool 10 starts tosink through the body 8. An indication of the soda content of the body 8can be obtained by monitoring the electrical conductivity of the block,which will increase as the soda content increases. The expectedoperational lifetime of a body 8 having a thickness of l inch is atleast 6 months.

Another example of a suitable refractory material for the body 8 is thatcommonly used to form the tank lining refractory blocks 6. This materialhas the advantage of having a higher electrical conductivity thansilica, although the latter has a greater capacity for absorbing soda.In any event the refractory body 8 should be capable of dissolving soda,that is, it should have a high silica rather than a high aluminacontent.

When the invention is applied to an apparatus for effecting surfacetreatment (e.g. tinting) of glass, the body 8 is effective in removingsodium metal from the bath 3, resulting from electrolytic migration intothe tin of the bath. In this case the sodium metal forms sodium ionswhich are removed from the surface of the bath by the electric field Eapplied to the body 8, even if the sodium metal has not been oxidized inthe bath to soda (Na O). It is not then necessary to use a refractorymaterial for the body 8 which dissolves soda. For the removal of sodiummetal, therefore, a refractory body 8 having a high alumina contentcould in principle be used.

The embodiment of FIG. 1 is illustrated in its application to the hotinlet end of the molten metal bath. It will be appreciated, however,that this embodiment is equally applicable to the cooler outlet end ofthe bath, where the temperature may be of the order of 750 C. In view ofthe lower temperature in this region the body 8 in this case need not beof refractory material but could, for example, be of glass.

The invention can also be applied to the lining blocks 6 of the tankdirectly. A direct current source is connected across the internal andexternal surfaces of the lining to produce an electric field in thelining directed from the interior towards the exterior of the lining, inwhich case the previously described mechanisms would operate to removean alkali metal oxide, for example, soda, from the interior surfaces ofthe lining. Glaze would then begin to form on the external, rather thanthe internal, surfaces of the lining, and the drawbacks associated withthe formation of glaze on the interior of a lining would again beobviated.

Where refractory lining blocks 6 are employed molten metal from the bath3 may penetrate the cracks in and between the blocks 6 and tend toprovide short-circuit paths between the internal and external faces ofthe blocks 6. This is obviated by connecting the negative terminal ofthe direct current source to an electrode inserted in a pocket drilledin the external face of a block 6.

An alternative embodiment of the invention is illustrateddiagrammatically in FIGS. 2 and 3, where the same reference numeralsindicate like parts. In this embodiment the molten metal (tin) from thebath 3 is circulated through a chamber 12 communicating with theinterior of the tank structure 1 through an inlet 14 and an outlet 15.Circulation of the metal through the chamber 12 is effected by a rotarypaddle-wheel pump 16 in the inlet 14.

The body 8 is disposed in the chamber 12 and has a lower flat surface incontact with the molten metal in the chamber. A pool 10 of molten tin onthe upper surface of the body 8 forms an effective cathode, and as inthe embodiment of FIG. 1 an electric field E is established in the body8 away from the surface in contact with the molten metal by connectingthe metal in the chamber 12 and the pool 10 to the positive and negativeterminals respectively of a direct current source 11.

In this embodiment the body 8 comprises glass or a refractory at atemperature between 700 C and l,000 C depending on the location of thechamber 12 along the bath. The body 8 of glass is contained laterally byan annular refractory wall 17.

A plurality of chambers 12 ofthe kind shown in FIGS. 2 and 3 may beprovided, the inlets and outlets of the chambers communicating with thetank structure 1 at respective locations spaced apart along the lengthof the tank structure.

The rate of operation of the pump or pumps 16 is adjusted so that thequantity of soda in the metal drawn into the chamber 12 in a given timeis equal to the quantity resulting from the glass ribbon 2 in that time.Since currents of up to amps at 100 volts are practicable, the need fora large interfacial area between the body 8 and the molten metal (tin)is avoided. Moreover the passage of a high current of this order throughthe body 8 heats the body and assists in maintaining it at the requiredtemperature.

We claim:

1. Apparatus for use at a temperature in the range of about 600 C tol,000 C in the production of float glass comprising a tank structurecontaining a float glass supporting bath of molten metal selected fromthe group consisting of molten tin and molten tin alloys in which tinpredominates, a solid body of siliceous material separate from saidfloat glass, said body of siliceous material having a face in contactwith said molten metal bath and said body of siliceous materialcontaining silica in an amount sufficiently high to dissolve alkalimetal ions from the bath, and a direct current source connected to thebath and to a face of the body not contacting the bath, the sense of theconnection being such that the molten metal bath is anodic therebycausing alkali metal ions to be drawn into the siliceous body from theinterface between the molten bath and the body.

molten metal bath which is not covered by the glass.

5. Apparatus according to claim 1, including means for circulating themolten metal of the bath through a chamber communicating with the tankstructure, the said body being disposed in said chamber in contact withsaid molten metal.

1. Apparatus for use at a temperature in the range of about 600* C to1,000* C in the production of float glass comprising a tank structurecontaining a float glass supporting bath of molten metal selected fromthe group consisting of molten tin and molten tin alloys in which tinpredominates, a solid body of siliceous material separate from saidfloat glass, said body of siliceous material having a face in contactwith said molten metal bath and said body of siliceous materialcontaining silica in an amount sufficiently high to dissolve alkalimetal ions from the bath, and a direct current source connected to thebath and to a face of the body not contacting the bath, the sense of theconnection being such that the molten metal bath is anodic therebycausing alkali metal ions to be drawn into the siliceous body from theinterface between the molten bath and the body.
 2. Apparatus accordingto claim 1, wherein the negative terminal of the source is connected toa pool of molten metal on the upper surface of the siliceous body. 3.Apparatus according to claim 1, wherein the said body is a block ofsilica-rich refractory material.
 4. Apparatus according to claim 1,wherein the said body is maintained in contact with a part of the uppersurface of the molten metal bath which is not covered by the glass. 5.Apparatus according to claim 1, including means for circulating themolten metal of the bath through a chamber communicating with the tankstructure, the said body being disposed in said chamber in contact withsaid molten metal.